Existing website content, URLs, media, branding, data, and code should be evaluated separately by usefulness, accuracy, ownership, portability, and maintenance cost rather than kept or discarded based on age alone.
Diagnose post-redesign search problems by comparing old and new URLs, redirects, indexability, internal links, page content, titles, and measurement before adding unrelated SEO work.
An old static site can be modernized by improving templates, semantic HTML, responsive layout, navigation, performance, and maintainability without adding a full CMS when editing needs are limited.
A bad redesign should be evaluated against the customer tasks, content, URLs, functionality, performance, and owner workflow that existed before it so the repair targets real regressions rather than aesthetics alone.